Add krb5_c_prfplus() and krb5_c_derive_prfplus()

Add krb5_c_prfplus() and krb5_c_derive_prfplus()

This commit permits the external use of the RFC 6113 PRF+ function.
It also adds a function to derive a key from an input key and string
using PRF+.

[ghudson@mit.edu: adjust style; avoid new C99isms; use string2data(),
empty_data(), and alloc_data() where appropriate; add some explanatory
comments; edit docstrings and commit message]

ticket: 8228 (new)